The present invention relates to a suspension system for motor vehicles, with hydraulic supporting units acting between the vehicle wheels and the vehicle body and with a hydraulic feed system arranged between the supporting units and operating under self-acting control, which system allows hydraulic fluid to be displaced between the supporting units in order to compensate, if required to overcompensate or to reduce pitching or rolling movements or tilted positions of the vehicle body.
In a system disclosed in German Patent No. 3,936,987, respective hydropneumatic supporting units are arranged between the vehicle wheels and the vehicle body, interacting with a relatively slow-acting level control system which, as a function of the lift position of the wheels, connects the supporting units via level control valves to a hydraulic pressure source in order to take up hydraulic fluid or to a reservoir to drain hydraulic fluid from the supporting units. An actively driven feed system is arranged in parallel with the level control system, between the supporting units, and allows hydraulic fluid to be displaced between supporting units of different wheels while bypassing the reservoir and the pressure source. In comparison with the level control system, this feed system is quick-acting and, in the event of a fault, in particular a fault in the associated computer, shuts down in a self-locking manner, blocking its lines. Any existing position compensation at the time of the occurrence of the fault is reduced by way of the level control system.
In this known system, hydraulic fluid is thus displaced directly between the supporting units on different sides of the vehicle or between the front and rear supporting units in order to raise one side of the vehicle while simultaneously lowering the other side of the vehicle or to move the front part of the vehicle in a corresponding manner in the opposite direction to the tail of the vehicle. It is thereby possible to counteract rolling and pitching movements. The power required for this purpose is low since all that is required is for the difference between the hydraulic pressures in the different supporting units to be overcome rather than the considerably greater pressure difference between the reservoir and the supporting units.
The level control system is slow-acting and accordingly likewise has a low power requirement. This system is not required to balance out pitching and rolling movements of the vehicle. Essentially, the level control system must merely be in a position to set the respectively desired or predetermined level within an acceptable time when the vehicle is started. In the event of a fault in the feed system, i.e. in particular, in the event of a fault in the associated electronic control, good emergency running characteristics of the vehicle are guaranteed.
All that is required is for the feed system to be disconnected from its energy supply by being cut off. The lines of the feed system or its feed device are thereby necessarily blocked. Even if this blocking occurs after the displacement of a relatively large quantity of hydraulic fluid between different supporting units, the vehicle remains fully under control since the level control system will, even if somewhat slowly, balance out the displacement of the hydraulic fluid again and, overall, ensure handling characteristics similar to those in vehicles with conventional hydropneumatic suspension systems without the feed system described above. This known suspension system is distinguished by good operating performance. However, due to the relatively high outlay on construction, it is only suitable for relatively expensive vehicles.
German Patent No. 3,644,931 discloses a similar suspension system with level control. German Offenlegungsschrift No. 3,502,579 and German Auslegeschrift No. 1,136,219 disclose mechanical suspension systems in which the spring elements are each arranged in series with hydraulic adjusting units which are used to adjust the level of the vehicle.
It is an object of the present invention to develop a system such that, on one hand, it is possible to use any desired suspension units even those with purely passive spring elements, for example helical springs or the like, in order to reduce the outlay on construction as far as possible. In addition, good emergency running characteristics should be achievable in the event of malfunctions, likewise with as low as possible constructional outlay.
The foregoing objects have been achieved according to the present invention by arranging the hydraulic supporting units in series with a spring unit acting between the wheel and the vehicle body and, in the event of a malfunction of the self-acting control, assuring that the units can be set to a central position or to a lowered end position by providing a hydrostatic feed system which, in the case of the malfunction, can be set to its (central) position, the position assigned to the central position, or the hydraulic units can be set to their lowered end position by being emptied.
Thus, according to the present invention, the hydraulic supporting units and the interposed hydraulic feed system form a system which is functionally separate from the spring units and which, in the event of a malfunction, can be shut down in a predetermined central position or end position, whereupon the handling characteristics are determined exclusively by the spring units and associated damper units.
According to a presently preferred embodiment of the invention, controllable shock dampers are provided, in each case in parallel with the spring units and in series with the supporting units. In this arrangement, the shock dampers cannot impair the action of the supporting units and of the feed system. In particular, this ensures that the feed system does not require any additional power to overcome the damping resistance of the shock dampers.
In a particularly expedient embodiment of the invention, it is possible to carry out control of the shock dampers independently of the self-acting control of the supporting units. The shock dampers can, for example, normally operate continuously with low damping and be switched to increased damping only if unwanted resonant vibrations, in particular in the event of so-called body resonance or of wheel resonance occur.
There is furthermore the advantageous possibility that the automatic control for the shock damper and the self-acting control for the supporting units interact, in such a way, for example, that pitching movements of the vehicle body are also counteracted by stiffening of the front or rear shock dampers of the vehicle and rolling movements are counteracted by stiffening of the shock dampers on one side of the vehicle. This offers the possibility of having the feed system operate comparatively slowly and counteracting abrupt pitching or rolling moments by way of the shock dampers.
The automatic control for the shock dampers can furthermore intervene in a compensating manner to a certain extent in the event of a malfunction of the self acting control for the hydraulic supporting units by virtue of the fact that the shock dampers are then also activated or activated to an increased extent in order to reduce pitching and rolling movements of the vehicle body.
In all these embodiments, it is advantageous that sensors for monitoring the positions or movements of the wheels or of the vehicle body can be used both for the work of the self-acting control for the supporting units and for the work of the automatic control for the shock dampers and that corresponding duplication is unnecessary.